The technical field of this invention is soldering and, in particular, soldering methods and compositions that resist conductive joint fracture and/or facilitate automated soldering of complex electronic components, such as chips to printed circuit boards, to other chips, or to other substrates. The invention further concerns solder compositions that exhibit compliance, i.e. the ability to give easily with strain, and thus resist conductive joint fracture.
Modem electronic devices are typically formed by soldering electronic components together. Such devices can include logic or memory chips on printed circuit boards, multichip modules or complex integrated circuits. In these devices, numerous electrical connections must be formed between a component and its mating substrate. The conventional approach, especially with simple electronic components such as resistors, capacitors, transistors and small chips, has been to pass leads from the component through holes in the substrate and then to fill the hole with solder to secure the connection. However, as the device structures become denser and chips become smaller and more complex, the use of leads has become more and more cumbersome.
A more recent approach to joining electronic components is xe2x80x9csurface mountingxe2x80x9d in which metal regions or tabs on the components are aligned and soldered to corresponding metallized pads on the substrate. In these devices, numerous connections must be formed between the component and conductive circuitry on the substrate (a circuit board). The conducting paths on a substrate may form a tight network or array of connecting points. Thus, accuracy and precision in placement of the conducting pads on the substrate is essential to proper function of the resulting device. In one common approach, often referred to as xe2x80x9cball grid arrayxe2x80x9d assembly, the metallized regions of the component or the mating pad of the substrate are prebumped with solder balls prior to assembly. The assembly is then heated so that reflow of the solder occurs, forming a permanent physical and electrical connection at each soldered point in the array.
Unfortunately, the reflow of molten solder is not always reliable. The solder can spread further than desired, causing short-circuits between adjacent conductive lines. In addition, the solder can migrate from the desired location creating a solder-starved joint or one in which the electrical connection has failed entirely. As the density of interconnective lines becomes greater and greater, the problem of solder reliability becomes increasingly more difficult to solve.
Moreover, most modem electronic solder connections are thin, high aspect ratio, joints. (The aspect ratio is a measure of the area of a solder joint divided by its thickness.) It is increasingly common for electronic components to be joined to printed circuit boards and the like by thin surface-mounting solder joints having a thickness of less than about 150 micrometers (less than 0.006 inches). In this regime, the solder joint is plastically constrained and can develop triaxial (hydrostatic) stresses several times greater than the average tensile strength of the bulk solder material. In addition, stresses arise from the thermal cycling of electronic circuits as the device incorporating such electronics is turned on and off. The solder joint experiences the full shear resulting from changes in component dimensions with temperature. Thus, even when solder joints are initially satisfactory, the nature of the joint itself becomes a critical point for device failure over time.
In addition to the physical limitations of conventional solder compositions in effecting reliable electronic connections, more and more concern has been expressed in recent years over the use of lead as one of the principal components in conventional solders. The toxicity of lead and the human body""s limited capacity to reverse lead poisoning, has fueled a public health movement to curtail any unnecessary use of lead. The nature of automated soldering processes is such that significant amounts of lead are released as vapors into the processing facility. These vapors necessitate protective measures for personnel and the scrubbing of air before it can be released to the ambient environment. Moreover, since every electronic device eventually fails and must be discarded, the use of heavily leaded solders places an additional burden on waste disposal programs.
There exists a need for better methods and compositions for joining electronic components which can reduce the likelihood of conductive joint failure during the initial soldering process, and/or inhibit subsequent joint fatigue and/or fracture. Methods and compositions that can improve the efficiency of assembling ball grid arrays and the like would provide a solution to a problem that has troubled the automated manufacturing of complex electronic devices for a long time. Likewise, soldering methods and compositions which reduce triaxial stresses and, hence, the potential for joint fatigue, solder fracture or other electronic failure of thin, high aspect, solder joints, generally, would satisfy a long felt need in the art.
Moreover, solder compositions which can reduce the amount of lead necessary to form a reliable electric connection, and/or reduce the release of lead during manufacturing or waste disposal, would also address a long unsolved problem.
Soldering methods and compositions are disclosed that provide electrical connections between surfaces with reduced likelihood of short circuits or solder-starved joints. In addition, solder compositions are disclosed that exhibit compliance, i.e. the ability to give easily with strain, and thus resist conductive joint fracture. These solder compositions preferably form xe2x80x9cmetallic foamxe2x80x9d joints upon heating.
Because the solder compositions of the present invention can fuse together rapidly without necessarily becoming entirely liquid, the invention reduces the likelihood of reflow-related solder joint faults. They can also reduce the potential for thermal fatigue and other solder joint failures in electronic devices following fabrication because the porous solder joints relieve plastic constraints and lower the average tensile joint stress. The compositions further provide proper standoff distance, the distance between the component and the substrate, allowing greater contact of the joint with the component. In other words, the compositions, by producing joints with proper standoff, prevent slumping of the joints away from the component.
In one aspect of the invention, solder compositions are employed which are composed of particles of a first metal coated with a second metal. The metals are chosen such that their individual melting points are higher than the melting points of the alloy or alloys formed when they are combined. Upon heating of such coated particles, melting occurs at the interfaces between the core materials and their coatings. The liquid so formed causes various particles to fuse together in a porous metal foam that provides a compliant electronic connection capable of withstanding thermal cycling with significantly lower failure rates. This soldering technique is particularly advantageous when applied to soldering of grid arrays and similar structures that facilitate mounting of chips to printed circuit boards, other chips or substrates, generally. The present invention can also be useful in reducing the total amount of lead and/or other toxic components present in solder compositions.
More generally, the solder compositions of the invention are formed from two distinct metallic components. The components can be elemental metals or metal salts. The components can be coated one upon the other or they can be physically separated. For example, an alternative to the bimetallic particles described above can be formed by particles of first metal surrounded by salt solution or suspension of the second metal. Again, the metals are chosen such that their individual melting points are higher than the melting points of the alloy or alloys formed when they combine. Upon heating to soldering temperature, the second metal or metal salt precipitates to coat the first metal particles with the second metal. Melting occurs at the interfaces between the core material, the first metal particles, and their coatings. Thus, by controlling the relative concentrations of the first metal and of the second metal, a porous metal foam is achieved.
High aspect ratio (large diameter/thickness) solder joints which are plastically constrained develop large hydrostatic stresses (Friction Hill) greatly in excess of their yield strength. Because the local high triaxial stresses arising from the Friction Hill prevent homogeneous yielding and, in a strain controlled system, will localize plastic deformation within the regions near free surfaces, abrupt brittle fracture through an intermetallic or along an interface can occur. In such situations, the service life of the joint during fatigue loading such as thermal cycling will be greatly reduced. The prevention of triaxial stress build up within such a strain controlled environment which can occur in, for example, leadless chip carrier solder joints requires a distribution of internal free surfaces within the joint. The solder system disclosed herein is a thin porous metal film with a regular distribution of pores.
In one embodiment, the solder composition can be formed from the usual components, tin and lead or their salts. For example, small lead or tin particles can be coated with a thin film of the other component, and mixed with flux paste. Alternatively, small particles of one metal, a metallic salt of the other metal and a flux can be employed. In either case when the temperature is raised to just above the eutectic temperature, solid state diffusion occurs across the lead-tin interface until its composition reaches the melting point. The particles then are interconnected by a thin near eutectic temperature liquid film. Capillary action draws the incompletely melted particles together. However, the particles do not flow because they are interlocked by this capillary action. Additional metal from the solid particle dissolves into the liquid increasing its composition and, thus its melting point. Diffusion into the liquid continues and its melting point rises until it solidifies isothermally. This forms an interconnecting network of solder xe2x80x9cmini-elementsxe2x80x9d with a dense pore structure. It should be noted that the surfaces of the particles first melt and then solidify at a constant temperature as their composition changes.
The invention can be practiced by forming spheres (or other shapes) of one metal and coating them with another metal. For example, lead spheres coated with a thin layer of tin, or tin spheres coated with a thin layer of lead can be employed. Pure tin melts at 232xc2x0 C. and pure lead melts at 327xc2x0 C., but the lowest eutectic point of tin-lead alloys is 183xc2x0 C. Hence, when the temperature of coated particles is brought to about 183xc2x0 C., the lead and tin dissolve into each other at the interface between the core and the coating, forming a liquid. This liquid wets and bonds the particles into a continuous but porous joint that resembles a metal foam.
More generally, solder compositions according to the invention can be formed from bimetallic particles having a first core metal component with a first melting point and a second coating metal component with a second melting point. This coating process can occur prior to use or during the soldering process. The first and second metals are chosen for their capability of forming an alloy having a third melting point that is lower than either the first or second melting points, such that upon heating a liquid is formed at the interface between said first and second components of the particles. The reflow of the molten liquid between adjacent particles fuses the particles together as the composition goes beyond the eutectic point and re-solidifies.
Various techniques, known in the art, can be used to formn the coated, bimetallic particles of the invention, including for example, plating techniques. Alternatively, other coating techniques, known in the art, such as electroplating, deposition/substitution in the presence of an organic acid, barrel plating, vapor deposition, chemical vapor deposition, sputtering and ion or electron beam-assisted deposition can be employed.
The particles can take various shapes, including spherical, solid polygonal, ring-like, hollow cylindrical, conical and arcuate (crescent, or xe2x80x9cC-clipxe2x80x9d or otherwise curved) shapes. The choice of particle shape will influence, to an extent, the porosity and compliance of the resulting joint. Generally speaking, spherical particles provide greater packing density and, hence, will yield more compact (less porous) joint structures. Other shapes can be employed when more porous joints are desired. Moreover, the use of xe2x80x9cC-clipsxe2x80x9d or otherwise arcuate shaped particles can be advantageously used to produce a joint with a higher degree of compliance and greater resiliency in response to vibrations. The metallic foams of the present invention, as well as the non-spherical geometries of the particles in some embodiments, facilitate more compliant joints and, thereby, allow the soldering of larger electronic components.
The size or diameter of the particles can also vary. (The term xe2x80x9cdiameterxe2x80x9d is used here in its common sense, e.g., the width of particle across its mean or largest dimension.) In some applications it can be desirable to have a range of particle sizes. For example, it can be desirable to have the particles vary in size such that the largest particles are larger by a factor of three vis-à-vis the smallest particles. In other applications, a narrower size distribution may be desired. In one preferred embodiment, the particles have an average diameter ranging from about 1.0 to about 5000 micrometers, more preferably ranging from about 5 to about 1000 micrometers, and most preferably ranging from about 10 to about 500 micrometers.
Low lead solder compositions can also be achieved with the bimetallic particles of the invention. Since solder reflow and fusion in the invention occur only in a thin interface between the core and coating, less lead is needed to form a joint. In one embodiment, lead-coated tin balls are disclosed which significantly reduce the overall lead content of the composition as compared with conventional (60%Sn - 40%Pb) solder compositions. In another application, lead core balls with relatively thicker tin coatings can be employed. Such tin-coated lead balls can be used to encapsulate the lead component of the solder in order to reduce vaporization of lead during soldering operations.
More generally, the coatings of the present invention can range from about 0.1 to about 1000 micrometers, more preferably ranging from about 1 to about 500 micrometers, and most preferably ranging from about 5 to about 100 micrometers. The solder composition can further comprises a plurality of particles which, upon fusion, yield a porous joint having an average pore size ranging from about 1 micrometer to about 1000 micrometers, preferably about 5 micrometers to about 500 micrometers.
The invention will next be described in connection with certain illustrated embodiments. However, it should be clear that various changes and modifications can be made by those skilled in the art without departing from the spirit or scope of the invention.